1. Field of the Invention
This invention relates to the art of control of a variable-speed reversible motor, and more particularly to an apparatus for controlling a motor arrangement including a variable-speed reversible main motor for driving a load and a subsidiary motor for driving a fan . 2. Description of the Prior Art
Generally, an electric motor connected to a heavy load to operate under a severe condition is arranged to be cooled by an external motor-driven blower. Further, it has been proposed a motor arrangement includes, besides a main motor, a fan-driving subsidiary motor which is disposed in coaxial relation with the rotary shaft of the main motor and is rotated independently of the rotation of the main motor, as disclosed in, for example, German Pat. No. 1,112,782. The fan-drive subsidiary motor of this kind can supply cooling air of constant amount regardless of the rotation speed of the main motor. Therefore, such a motor arrangement is preferably mounted on, for example, a battery-driven forklift truck which is repeatedly started and stopped for loading and unloading operation within a limited traveling range. However, in the case of a conventional battery-driven forklift truck, the external motor-driven blower starts to rotate at a constant speed in response to the turning-on of the key switch, regardless of whether or not the main motor is under rotation. More precisely, the motor-driven blower is energized to rotate as soon as the key switch is turned on at whatever position of the accelerator switch and is not deenergized unless the key switch is turned off, whereas the main motor would not be energized unless the key switch is turned on, and, then, the accelerator switch is turned on. This means that the fan is driven more than is required. Therefore, the vehicle, whose power source is the battery, has been encountered with the problem that the battery is considerably consumed, and the movable range of the vehicle is reduced. Further, the unnecessary drive of the fan, especially, under a cold condition or in a cold season has resulted in excessive cooling of the main motor, which is not desirable from the aspect of saving of energy too. Furthermore, the vehicle driver had to pay vigilant attention to the operating conditions of the main and subsidiary motors since, in the event that the external motor-driven blower or the fan-drive subsidiary motor arranged in coaxial relation with the main motor is disabled and rendered unable to perform its function for some unknown reason, the ambient temperature of the main motor shows an unusual rise until finally the main motor itself is rendered utterly unable to perform its primary function.